This invention generally pertains to the field of materials which closely mimic the mechanical characteristics of human tissue and organs, such as breast and prostate. More specifically, these materials can be used advantageously in manufacturing of tissue models, which are beneficial for training clinical practitioners to conduct palpatory examination as well as for testing and calibrating elasticity imaging devices.
Human tissue phantoms, in general, are used as tools for the assessment and verification of performance standards in the daily clinical practice of diagnostic radiology and ultrasonic equipment.
The U.S. Pat. No. 5,805,665 discloses an apparatus and a method of use and construction of anthropomorphic mammography phantoms, which provide mammography practitioners with a training tool to practice proper patient breast positioning and optimal patient X-ray exposure. The mammography phantoms simulate normal breast tissue as well as tissue irregularities and anomalies associated with various known breast pathologies. Examples of such pathologies include microcalcifications, cysts, tumors, etc. The mammography phantoms preferably conform to the shape of the upper torso of a female, complete with one or two breast simulators that can vary in size, density, compressibility, and stretchability.
One example of a phantom for the use in training of practitioners to locate and aspirate tumors identified by X-ray consists of a bulk of tissue-equivalent, X-ray permeable material containing at least one simulated tumor as described in U.S. Pat. No. 5,273,435. A phantom can be fabricated from an X-ray permeable material (such as gelatin with a radiopaque material like iodinated oil containing lampblack) poured into a mold with the shape of a human breast.
Another example is an ultrasound phantom containing tissue-mimicking material and disclosed in U.S. Pat. No. 4,277,367, in which both the speed and attenuation of sound could be simultaneously adjusted using water based gels like those derived from animal hides. In one embodiment, ultrasound phantoms with desired ultrasonic properties are prepared from a mixture of gelatin, water, n-propanol, and graphite powder—all mixed with a preservative. In another embodiment, an oil and gelatin mixture forms the basis of the tissue-mimicking material.
A tissue-mimicking material for use in ultrasound applications possessing the ultrasonic speed and attenuation characteristics of human tissue is described in U.S. Pat. No. 5,902,748. This material includes a pure gel-forming component and hydroxy compound, such as an n-propanol, to adjust the ultrasonic speed of propagation through the material. The tissue-mimicking material may be included in an ultrasound phantom container with solid scattering particles and/or test objects incorporated therein.
A breast-shaped phantom for optical and magnetic resonance imaging quality control is known from U.S. Pat. No. 6,675,035. This phantom consists of a cup in the shape of a human breast in its natural pendant position and a filler (agar, fat emulsion) occupying the volume of the cup. The cup forms an outer skin (polyurethane) of the phantom with a thickness similar to human skin and with optical transparency at selected optical wavelengths similar to human skin. The filler material has optical scattering and absorption characteristics similar to human breast tissue.
An interactive breast examination training system to familiarize physicians and other medical personnel with the techniques for clinical breast examinations is described in U.S. Pat. No. 6,945,783. The system includes a model with an outer shape comparable to a human breast. Inflatable nodules are embedded at various locations and depths in the model. The nodules are adapted by being inflated to simulate tumors and are relatively undetectable by touch when deflated. Pressure sensors are fluidly coupled to the nodules to sense fluid pressure in the nodules. A pump is fluidly coupled to the nodules to inflate the nodules. A processor is operatively coupled to the pressure sensors and the pump to inflate the nodules to desired hardness levels based upon pressure readings from the pressure sensors.
It is clear that tissue-mimicking materials must exhibit the same properties relevant to a particular imaging modality as those of actual human soft tissues. For elasticity imaging, tissue-mimicking materials for use in phantoms should have mechanical properties that correspond to those of real tissue. Soft tissues have Young's modulus values ranging from about 2 kPa for normal tissue to about 1,000 kPa for pathological anomalies such as cancer. Tissue-mimicking materials for use in magnetic resonance imaging phantoms should have values of characteristic relaxation times, T1 and T2, which correspond to those of tissue. Soft tissues exhibit T1 values ranging from about 200-1200 ms and T2 values from about 40-200 ms. The tissue-mimicking material for use in ultrasound should have the same range of speeds of sound, attenuation coefficients, and backscatter coefficients as soft tissue. These parameters should be controllable in the manufacturing process of the phantom material, and their variation within the range of room temperatures should be small. Speeds of sound in human soft tissues vary over a fairly small range with an average value of about 1540 m/s. The speed of sound in fat is thought to be about 1470 m/s. The amplitude attenuation coefficients in these tissues appear to vary over the range from about 0.4 dB/cm to about 2 dB/cm at a frequency of 1 MHz. For use in computed tomography (CT), the tissue-mimicking materials must exhibit the same CT number as that of the tissue being mimicked. The CT numbers for most soft tissues lie in the range of about 20-70 at the typical effective X-ray energy of a clinical CT scanner, except for fat where the CT number is about 100. Although known tissue phantoms accurately mimic ultrasonic, X-ray, optical and MRI related parameters within the range characteristic for tissues in vivo, there are no tissue phantoms accurately representing entire range of the elasticity modulus for normal and diseased tissue.
There is a need for phantoms that emulate the real mechanical characteristics of human tissue, (such as the breast and prostate as examples) that resemble an organ in shape and size appropriate for use in training clinical practitioners conducting manual palpation.
There is also a need for phantoms with predetermined elasticity properties representative of the entire range of normal and pathological tissue for calibrating and testing elasticity imaging devices.